This invention relates to a sheet feeding apparatus adapted to feed sheets to an image forming machine such as a printer, and more particularly to a sheet feeding apparatus which are able to stably feed different kinds of sheets which is different in paper quality such as thickness and hardness.
FIG. 9 is a side view showing the arrangement of a conventional sheet feeding apparatus 50 which is adapted to feed a number of sheets P to an image forming section (not shown) one after another. The unit 50 has a sheet feeding table 51 on which sheets are stacked. The table 51 is a conventional one which is vertically moved by driving means (not shown).
In the operation of the image forming machine, first the sheet feeding table 51 is moved downwardly, and sheets P are stacked on it. Under this condition, the sheet feeding table 51 is moved upwardly. The stop position where the sheet feeding table 51 thus moved is stopped is detected by a sensor 52 adapted to detect the position of a sheet feeding roller mechanism 53.
The sheet feeding roller mechanism 53 is swingable about a shaft 53a. Hence, as the sheet feeding table 51 is moved upwardly, the sheet feeding roller mechanism 53 is lifted through the stack of sheets P, so as to activate the sensor 52 thereby to stop the upward movement of the sheet feeding table 51.
Thus, the sheets P are supplied into the machine one after another by means of a scraper roller 54a and a pick-up roller 54b in the sheet feeding roller mechanism.
However, the above-described sheet feeding apparatus suffers from a difficulty that it is difficult for the unit to accurately feed sheets which are different in thickness or in hardness; more specifically, sometimes no sheet is supplied to the machine or more than one sheet are supplied to it at the same time.
In the case of FIG. 9, the sheet feeding apparatus is so set that the positional relationship between a stack of hard sheets P1 and the sheet feeding roller mechanism 53; that is, the top of the stack of sheets P1 is in contact with the scraper roller 54a under a predetermined pressure, while it is spaced a certain distance from the pick-up roller 54b.
However, the sheet feeding apparatus in this state is not applicable for feeding soft sheets P2; that is, it is difficult for the sheet feeding apparatus under this condition to suitably feed the soft sheets P2.
A stack of soft sheets P2 are flexible when compared with a stack of hard sheets, and therefore as shown in FIG. 10 the scraper roller 54a of the sheet feeding roller mechanism 53 sinks in the stack of soft sheets P2 as much, and accordingly when the sheet feeding table 51 is moved above the stop position shown in FIG. 9, the sensor is activated. That is, in the case of FIG. 10, the stop position is located above the stop position in the case of FIG. 9.
Hence, the top of the stack of the soft sheets P2 is brought into contact with the pickup roller 54b under a pressure, too, as a result of which more than one sheet P2 may be often supplied into the image forming machine at the same time.
On the other hand, if hard sheets P1 are supplied under the same conditions as the soft sheets P2 have been supplied, that is, with the sheet feeding roller mechanism set in the same manner, then another difficulty is involved. The hard sheets P1 are not flexible, and therefore before the sheet feeding table 51, being lifted, reaches the stop position determined for the feeding of soft sheets P2, the sensor 52 is activated, so that the sheet feeding table 51 is stopped below the most suitable position (or the stop position for the feeding of hard sheets).
In this case, the top of the stack of the hard sheets P1 is spaced from the pickup roller 54b of the sheet feeding roller mechanism 53. On the other hand, the scraper roller 54a tries to move (feed) the hard sheet P1, but cannot give it to the pickup roller 54b, merely pushing the end of the hard sheet P1 against the side board 56a of the guide board 56; that is, no hard sheet is supplied into the image forming machine.
In general, a hard sheet P1 is large in thickness, while the soft sheet P2 is small in thickness. Hence, if those two kinds of sheets, namely, hard sheets and soft sheets are supplied into the machine with one and the same sheet feeding apparatus 50, the above-described difficulty occurs with the feeding of hard sheets or with the feeding of soft sheets.
Accordingly, an object of the invention is to eliminate the above-described difficulties accompanying a conventional sheet feeding apparatus. More specifically, an object of the invention is to provide a sheet feeding apparatus which can handle both hard sheets and soft sheets and is free from difficulties that no sheets is supplied into an image forming machine and more than one sheet is supplied thereinto at the same time.